As data communication services continue to develop and their uses continue to expand, the demand for higher transmission rates becomes greater. Similarly, the ability to satisfy this demand becomes more important.
One particularly promising technique for providing high speed data connections between a terminal and the subscribers served by that terminal is a passive optical network (PON). PON service can provide extremely high speeds compared to services provided using standard coaxial cable and digital subscriber lines (DSL), and can be installed at a cost substantially below that of conventional optical networks. A passive optical network delivers data to users through a connection between an optical line termination unit (OLT), which may suitably form a data communication terminal at which data is transmitted and received by a data provider, and a plurality of optical network units (ONUs). An ONU may be installed at a subscriber's home or business and may serve as the communication terminal whereby the subscriber receives data from and transmits data to the data provider. The connections between the OLT and the ONUs do not include any active electronics, but simply provide optical pathways between the OLT and the ONUs served by the OLT. Therefore, the connections between the OLT and the ONUs can be deployed relatively inexpensively.
Typical prior art passive optical networks support a fixed bit rate between each OLT and a group of ONUs connected to the OLT, typically predetermined by deploying an OLT and ONUs having desired characteristics, including the desired fixed bit rate operation. Each OLT and ONU includes a clock and data recovery circuit (CDR) that receives and interprets data signals received over the passive optical network. Each CDR receiving a signal locks to the bit rate of the received signal when the received signal exhibits the expected bit rate. However, the CDR falls out of lock when the signal does not exhibit the expected bit rate. The rate at which the OLT transmits to one or more ONUs is referred to as the “downstream” bit rate and the rate at which an ONU transmits to the OLT is referred to as the “upstream” bit rate. Prior art passive optical networks require that an OLT transmit at a single downstream bit rate to a group of ONUs to which it is connected and that the ONUs all share a common upstream bit rate.
Prior art passive optical network systems do not allow transmission at different bit rates between an OLT and a plurality of ONUs to which the OLT is connected. Therefore, these systems require a set of unique ONUs and OLTs for each bit rate that is to be supported, and connections between the OLTs and the ONUs for each bit rate. This limitation requires considerable duplication of infrastructure, because an OLT and accompanying cabling must be provided for each downstream bit rate that is to be supported. A group of customers desiring service at different bit rates cannot be served by the same OLT, even if the number of customers is within the capacity of the OLT and the customers are all located within range of the OLT. In addition, the need to transmit and receive at a single fixed bit rate reduces the flexibility of a data service, because it is difficult or impossible to change bit rates to adapt to changing needs or conditions, such as changes in bit rates required by particular customer applications or changes in bit rates supported by prevailing conditions affecting an optical path, for example.
There exists, therefore, a need for a passive optical network that provides for transmission at different bit rates between an OLT and a plurality of ONUs to which the OLT may be connected.